FIG. 10 shows the adaptive control unit for an electrical discharge machine disclosed in Japanese Patent Publication No. 10769/1987. This adaptive control unit includes a machining electrode 1, a workpiece 2, a machining tank 3 containing dielectric material 4, a spindle 5 for moving the machining electrode 1 in a vertical direction, e.g., a Z axis direction, a drive motor 6 for driving the spindle 5, and a speed/position detector 7 for detecting the speed and position of the spindle 5. The adaptive control unit further includes an electrode position control circuit 21 for providing a drive command to the drive motor 6 which controls the position of the machining electrode 1, a machining power supply circuit 22 for supplying a machining voltage across the machining electrode 1 and workpiece 2, a detected value processor 23, coupled to receive a detection signal from the position detector 7 and coupled to receive the machining gap voltage provided by the supply circuit 22, for providing the electrode position control area 21 and the machining power supply circuit 22 with feedback commands, and for providing an adaptive control circuit 31 with a command signal to raise a bottom point of the electrode 1, i.e., the point where the downward movement of the machining electrode 1 towards the workpiece 2 changes and the electrode is driven upward away from the workpiece, and for providing the machining power supply circuit 22 with a machining command.
The adaptive control area 31 provides the electrode position control circuit 21 and the machining power supply circuit 22 with machining commands to perform adaptive control in accordance with an output signal received from the detected value processor circuit 23.
The operation of the control unit shown in FIG. 10 will now be described.
When a pulse voltage is applied by the machining power supply circuit 22 between the machining electrode 1 and the workpiece 2, an electrical discharge occurs therebetween in the dielectric material 4. As a result of this electrical discharge, and of the feeding operation of the electrode 1, the workpiece 2 is machined. In order to maintain an appropriate gap for electrical discharge between the machining electrode 1 and the workpiece 2, the electrode position control area 21 compares an average machining gap voltage provided by the detected value processor circuit 23 with a reference voltage, and controls the drive motor 6 in accordance with the results of this comparison in order to control the position or feed rate of the machining electrode 1.
In electrical discharge machining, the gap between the machining electrode 1 and workpiece 2 is on the order of between ten microns or several tens of microns. When the area to be machined is relatively large, it is difficult for cuttings generated from the large machining area to be ejected through the machining gap, i.e., between electrode 1 and workpiece 2. If cuttings remain in the machining gap, then the electrical discharge will concentrate on that area, thereby resulting in a faulty electrical discharge condition. This faulty discharge condition occurs when the amount of cuttings from the workpiece has surpassed the ejection capability of the cuttings. The faulty discharge condition can be prevented by detecting or estimating when the faulty condition will occur, and then reducing the amount of cuttings or improving the ejection capability. The occurrence of the faulty discharge condition will now be described further in connection with FIGS. 11(a) and 11(b), each of which depicts the movement of the machining electrode 1.
FIG. 11(a) represents a normal machining operation, whereas FIG. 11(b) represents a faulty machining operation. During machining, the machining electrode 1 vibrates up and down by an amount between about ten microns and several tens of microns. As normal machining progresses, the bottom point 101 (FIG. 11(a)), i.e., the point where the falling machining electrode 1 begins to rise, gradually lowers. However, if a faulty condition occurs in the machining gap, the bottom point 101 starts to move upward, as shown in FIG. 11(b). In order to prevent the faulty condition, it is necessary to shorten the width of the current pulse supplied by the machining power supply circuit 22 or to prolong a discharge stop width in order to stem the rise of the bottom point 101 and minimize the amount of cuttings produced. Alternatively, it is necessary to increase the periodic raising height of the machining electrode 1, i.e., the height by which the electrode is periodically separated from the workpiece during electrical discharge machining, in order to improve the ejection capability of the machined cuttings. Increasing the raising frequency is also effective.
In FIG. 10, the detected value processor circuit 23 detects the bottom point 101 in accordance with the motion of the machining electrode 1 obtained from the position detector 7, and provides a signal to the adaptive control circuit 31 which indicates the rise or fall of the bottom point 101. When a detected rise of the bottom point 101 has exceeded a predetermined threshold value, the adaptive control area 31 determines that a fault has occurred in the machining gap, and in response to this fault determination controls the power supply circuit 22 and the electrode position control circuit 21 to shorten the current pulse width and to prolong the electrical discharge stop width in order to minimize the amount of cuttings produced. Alternatively, the adaptive control area circuit 31 commands the electrode position control circuit 21 and the machining power supply circuit 22 to increase the periodic electrode raising value in order to improve the ejection capability of the machined cuttings.
Consequently, according to the prior art device discussed above it is necessary to provide a control scheme for shortening the current pulse width or for increasing the periodic raising value of the machining electrode 1 in order to prevent a faulty electrical discharge. However, it is difficult to achieve such a control scheme using an accurate technique because the periodic electrode raising value simply increases when the rise of the bottom point has exceeded a predetermined threshold value. Specifically, a skilled operator of the control unit judges the instability of an electrical discharge condition from various motions of the machining electrode 1 and from data representing an electrical value across the machining electrode 1 and workpiece 2, and changes the electrical discharge width and periodic electrode raising value in accordance with the judged instability.
In controlling the current pulse width and periodic electrode raising value automatically in accordance with the instability of the electrical discharge machining condition, instead of using the control provided by a skilled operator, it is difficult to describe properly through electronics or software the judgment criteria of the skilled operator.
To solve the problem of accurately emulating the judgment provided by a skilled operator, an adaptive control unit for an electrical discharge machine was developed by the present applicant in Japanese Patent Application No. 189844/1988. According to this adaptive control unit, a storage area is provided wherein a technique for recognizing a machining state is stored. This technique detects a machining state using a detected value processor, stores that detected state in a state storage area, synthesizes a plurality of results provided by the technique from a knowledge storage area and the machining status from the status storage area, by means of an inferring area, to obtain the machining state or a value which is equivalent thereto, and defines a manipulated value using that state or value.
The adaptive control unit allows the techniques of the skilled operator or the judgment criteria, etc., of the operator as to machining condition changes and electrical discharge machining state instability to be described properly and easily. These techniques permit optimum machining conditions to be executed and adaptive change to be automatically made.
However, when the area to be machined is large, in addition to those unstable states described above, the machining state becomes instable for a short time when the machining electrode and workpiece approach each other and machining is started and thereafter the machining state may become stable or may remain instable until the machining electrode and workpiece are separated. This instability can be corrected by reducing the electrode raising/lowering speed, which decreased the efficiency of the electrical discharge machining. Presently, a skilled operator monitors the instability of the electrical discharge machining state and alters the operation in accordance with the monitored instability
As described above, the prior art adaptive control unit for an electrical discharge machine detects a machining state value in a predetermined section, synthesizes a plurality of results from the detected machining state value and from techniques stored in a knowledge storage area in order to determine the electrical discharge machining state. However, with this adaptive control unit the instability of the electrical discharge machining state from when machining is initially started to when the electrode is separated from the workpiece cannot be properly determined. Further, the prior art adaptive control unit cannot properly determine if the unstable state occurs only momentarily at the beginning of machining. For this reason, even if the electrical discharge machining state improves after the beginning of machining, or if an unstable state occurs only momentarily, the machining state is recognized as instable, and adaptive control corresponding to the level of a skilled operator cannot be achieved. This is especially true when the area to be machined is large. Specifically, in the case of a large machining area, the control of the electrode raising/lowering speed, the electrical discharge stop/pulse, and the electrode raising value/lowering period, etc., cannot be easily differentiated if the machining conditions are controlled with the teohniques of the prior art adaptive control unit.